Locomotives that are used for heavy haul applications typically operate at higher altitudes and thus experience extreme environmental conditions, such as sub-freezing temperatures, low atmospheric pressure and higher than normal cosmic radiation exposure. This is because the higher the altitude of the locomotive, the higher the exposure to cosmic radiation, the lower the ambient temperature and the lower the atmospheric pressure. One disadvantage to operating at higher altitudes involves the increased exposure of the locomotive to cosmic radiation. AC locomotives typically use power semiconductors, such as Integrated Gate Bipolar Transistors (IGBT) or Gate Turn-Off (GTO) Thyristors, to control the power flow to the traction motors. Unfortunately however, these types of power semiconductor devices are adversely affected by cosmic radiation and this higher than normal cosmic radiation exposure tends to cause these power semiconductor devices to fail at a higher rate. Although the failure rate depends upon various other factors as well, such as voltage stress and temperature, exposure to cosmic radiation is a large contributing factor to power flow device failure as shown in FIG. 1 which shows an example curve illustrating the failure rate of the semiconductor devices as a function of altitude. For example, locomotives operating at altitudes of 5000 meters will have a significantly higher failure rate (approximately 25 times) than that of locomotives operating at sea level.
Another disadvantage to operating at higher altitudes involves the lower than normal air density (as compared to the air density at sea level). This is because at higher altitudes the air density decreases, therefore at high altitudes a higher volume of air is required to deliver the same mass flow rate of air to cool the traction motors and/or other equipment as is required at sea level. Unfortunately however, in order to generate the required higher volume of air to achieve the required air mass flow, a larger (in both size and weight) than normal blower and associated ventilation system is required and acts to increase the total weight of the locomotive. This is undesirable because the total weight of the locomotive may be limited due to infrastructure considerations. For example, referring to FIG. 2, a prior art traction system 400 for a typical locomotive is shown and includes six (6) prior art traction motors 402 and two (2) prior art blowers 404, wherein one (1) of the prior art blowers 406 is associated with three (3) of the prior art traction motors 408 and the other of the prior art blowers 410 is associated with the remaining three (3) of the prior art traction motors 412.
Thus, this traction system 400 requires two (2) medium sized motor blowers 404 to cool the smaller sized traction motors 402, wherein these blowers 404 are relatively small and therefore can be operated at lower altitudes. Unfortunately however, operation at higher altitudes may stress the performance of these blowers and add to the failure rate of the semiconductor components. As such, if the locomotive is operated at lower altitudes the cosmic effects are much lower than at higher levels and the combined effects of the DC bus voltage and the cosmic radiation on the semiconductor electric power control component does not typically exceed any of the parameters of the semiconductor electric power control components and the failure rate of these components is low. However, if the locomotive is operated at higher levels the cosmic effects are much higher than at lower levels and the combined effects of the DC bus voltage and the cosmic radiation on the semiconductor electric power control component tends to approach and/or exceed the parameters of the semiconductor electric power control components resulting in a higher than normal component failure rate.
An additional problem of locomotive operation at higher altitude involves the low ambient air density and the associated weight increase of the blowers due to the additional work required to produce an adequate amount of airflow needed to cool the equipment, such as the traction motors. One way to address this problem may be to allow the traction motors to be operated at higher temperatures. Unfortunately, this is undesirable because these higher operating temperatures may add significantly to the failure rate and/or insulation costs. Thus, as the altitude decreases the power required to move the same volume of air increases. Blowers typically move the same volume of air at a given speed. Blowers used on locomotives run at a fixed speed (typically proportional to the engine speed). Therefore, the increased sized blowers that can produce the required air flow at high altitudes, generally cannot be operated at the same speed at lower altitudes (such as sea level) due to the stresses produced in the blower and the increased horsepower required by the blower motor.